The present invention relates to an active matrix type liquid crystal display using a ferroelectric liquid crystal or antiferroelectric liquid crystal.
Active matrix type liquid crystal displays using TFTs (Thin Film Transistors) as switching devices are widely used in personal computers, word processors, television sets, and the like. Recently, with increasing sizes of display panels of active matrix type liquid crystal displays, 21-inch display panels have been developed. Accordingly, demands for improvements of the resolution of display images are increasing.
To realize large-sized display screens and high-resolution display images, a liquid crystal material having a wide angle of view and a high response speed is required. As a liquid crystal material meeting these requirements, ferroelectric liquid crystal or antiferroelectric liquid crystal having a higher response speed and a wider angle of view than those of conventional twisted nematic liquid crystal has attracted attention.
It is known that when liquid crystal having spontaneous polarization such as a ferroelectric liquid crystal or antiferroelectric liquid crystal (representatively liquid crystal having a chiral smectic C phase or its secondary phase), is driven by using TFTs, if the write time is shorter than the response time of the liquid crystal, the holding voltage drops due to the influence of an antielectric field (Hartmann: J. Appl. Phys. 66, 1132 (1989)). This phenomenon of the holding voltage drop is called "insufficient write". The phenomenon decreases the effective applied voltage with respect to the liquid crystal and causes a decrease in the contrast ratio. Therefore, the phenomenon is a serious problem when these liquid crystal materials are used.
For example, when a liquid crystal is driven in an active matrix type liquid crystal display having a conventional array structure shown in FIG. 8A, if the liquid crystal does not completely respond within one horizontal period, accumulated charge is canceled by an inverting current produced inside a pixel by the motion of liquid crystal molecules. Consequently, as shown in FIG. 8B, the holding voltage is lowered (J. Appl. Phys. 66(3), Aug. 1, 1989). This phenomenon is a serious problem in the case of ferroelectric liquid crystal having memory properties (SSFLC: Surface Stabilized Ferroelectric Liquid Crystal), because the transmittance of the liquid crystal changes. As described above, brightness changes when a ferroelectric liquid crystal which has an insufficient response speed, e.g., which does not completely respond within one horizontal period, is used in an active matrix type liquid crystal display having a conventional array structure.
Also, as another problem of the use of a ferroelectric liquid crystal or antiferroelectric liquid crystal, it is known that when the absolute value of a signal voltage changes from a certain frame period, a liquid crystal cell repetitively brightens and darkens over several frame periods and settles in a steady transmitted light quantity (this phenomenon is called "step response") (Verhulst et al.: IDRC '94 digest, 377 (1994)).
The above problem is solved when the response speed is made shorter than the write time by using a liquid crystal material with a sufficient response speed. At present, however, no liquid crystal material is known which satisfies this condition when a liquid crystal is driven at a low voltage or temperature slightly lower than room temperature. In particular, it is considered doubtful whether a high response speed at low temperatures is realized even in the future. Additionally, to accomplish a large screen and a high resolution of a liquid crystal display, it is essential to shorten one horizontal period. Therefore, it is difficult to solve the above problem only by improving a liquid crystal material.
As one solution for the step response, a method is known which performs a reset operation for writing 0 V in a pixel electrode immediately before a signal is written. Several variations of this method using a TFT or a TFD (Thin Film Diode) have been proposed. In these methods, a part of the write time is assigned to the reset operation. This solves the problem of the step response. However, the effective write time is reduced, and this worsen the problem of the insufficient write. Consequently, the contrast ratio cannot be satisfactorily improved.
The problem of the insufficient write is worsened when one horizontal period is shortened to realize a high resolution and at the same time the write time is further shortened by the reset operation. Note that if TFDs are used, the reset operation can also be performed while another line is being written. However, TFDs are unsuitable to be put into practical use because it is difficult to decrease variations of the device characteristics in one display panel.
As described above, when a liquid crystal material consisting of a ferroelectric liquid crystal or antiferroelectric liquid crystal is used, no conventional array structures can solve the problem of the insufficient write resulting from an antielectric field and the problem of the step response. Also, the conventional reset methods worsen the problem of the insufficient write resulting from an antielectric field, although they can eliminate the problem of the step response.